2 4 M ar 1 99 8 Low - Lying Energy Properties of a Frustrated Antiferromagnetic Spin - 1 2 Ladder

Using the Density Matrix Renormalization Group method, we determined the phase diagram of a frustrated antiferromagnetic spin2 ladder at zero temperature. Two spin-gapped phases, the Haldane phase and the singlet phase, are identified. A phase transition between the two phases occurs at any non-zero value of frustration coupling J×. On the phase boundary, the spin gap vanishes for sufficiently small J×. Crossing this non-gapped line, the transition is of second order, while of first order for larger J×. Striking frustration effects are predicted for ladder materials. PACS. 75.10.Jm, 75.30.Kz, 75.45.+j Typeset using REVTEX